Photoemissive tubes

Photoemissive tubes are necessary for work in the ultraviolet range and they show greater sensitivity and precision than photoelectric cells. A simple photo-emissive tube consists of two electrodes in a vacuum. A silver cathode coated with an alkali metal is maintained at a potential difference of about 100 V from the anode, which is a plain silver wire and serves to collect the electrons (Figure 2.26(a)). 

The spectral response of photoemissive tubes depends upon the composition of the cathode and the use of various mixtures of elements permits the production of a wide range of tubes of varying responsiveness (Figure 2.27). Photomultiplier tubes can be used to detect low intensity radiation and even in the absence of any light will still generate a small current due to various emissions from the material of the tube, etc. This dark current has to be compensated for in any measurements that are made. 

Figure 2.26 Photoemissive tubes. Light enters a simple phototube (a) and causes the release of electrons from the photoemissive alloy of the cathode. Owing to the potential difference between the anode and the cathode, the electrons are captured by the anode and the resulting current can be amplified and measured. Photomultiplier tubes (b) are a development of simple phototubes and result in internal amplification of the current initially developed at the photocathode.

Figure 2.27 Spectral response of cathode elements in photoemissive tubes.

Figure 8.16 Diagram of a photoemissive tube. R stands for resistance...

Regarding its design, a photoemissive detector may be a vacuum tube, photomultiplier (a tube plus dynode converter), some types of camera tubes, image converters, image amplifiers, etc. The main shortcoming of photoemissive tubes is the limited choice of cathode materials, which does not permit their maximum wavelengths to reach the spectral ranges of interest. We mention them for the sake of completeness, as well as because future solutions could potentially overcome this problem. 

Electronic Applications. Electronic appHcations make up a significant sector of the cesium market. The main appHcations are in vacuum tubes, photoemissive devices, and scintillation counters (see Electronic materials). 

The first photoelectric fhiorimeter was described by Jette and West in 1928. The instrument, which used two photoemissive cells, was employed for studying the quantitative effects of electrolytes upon the fluorescence of a series of substances, including quinine sulfate [5], In 1935, Cohen provides a review of the first photoelectric fluorimeters developed until then and describes his own apparatus using a very simple scheme. With the latter he obtained a typical analytical calibration curve, thus confirming the findings of Desha [33], The sensitivity of these photoelectric instruments was limited, and as a result utilization of the photomultiplier tube, invented by Zworykin and Rajchman in 1939 [34], was an important step forward in the development of suitable and more sensitive fluorometers. The pulse fhiorimeter, which can be used for direct measurements of fluorescence decay times and polarization, was developed around 1950, and was initiated by the commercialization of an adequate photomultiplier [35]. 

The electronic interaction of the relatively large molecules of phthalocyanine shows (Fig. 30) a considerable temperature effect (77a). In an experiment demonstrating this effect, the platinum foil (B in Fig. 2) was covered by the dye molecules until the work function was lowered to 4.32 volts at room temperature. If B was cooled by pouring liquid air into the upper tube of the photocell (a in Fig. 30), the photoelectric sensitivity increased and remained constant as long as liquid air was added. If the liquid air evaporated (6 in Fig. 30), the photoemission dropped to the original value at room temperature. This effect was arbitrarily reproducible. The calculation of the work function 4> and the constant M by the curves of Fowler [see Equation (5) in section III,la] in Fig. 31 gives = 4,32 volts, log M = —12.17 at room temperature (curve I), and = 4.15 volts, log M = —12.17 at low temperature (curve II). While... 

Vidicon. Although there are several types of vidicon tubes presently available, the most promising of these for spectroscopic work is the silicon vidicon, first conceived at Bell Labs (68). Figure 3 shows a diagram of a silicon vidicon. In contrast to a photomultiplier, which is based on a photoemissive principle, the vidicon television camera tube is based on a conductivity principle, a circumstance which explains its name. 

Flame photometry (see also p. 168) is almost exclusively used for the determination of alkali metals because of their low excitation potential (e.g. sodium 5.14eV and potassium 4.34 eV). This simplifies the instrumentation required and allows a cooler flame (air-propane, air-butane or air-natural gas) to be used in conjunction with a simpler spectrometer (interference filter). The use of an interference filter allows a large excess of light to be viewed by the detector. Thus, the expensive photomultiplier tube is not required and a cheaper detector can be used, e.g. a photodiode or photoemissive detector. The sample is introduced using a pneumatic nebulizer as described for FAAS (p. 172). Flame photometry is therefore a simple, robust and inexpensive technique for the determination of potassium (766.5 nm) or sodium (589.0nm) in clinical or environmental samples. The technique suffers from the same type of interferences as in FAAS. The operation of a flame photometer is described in Box 26.2. 

The very high reactivity of the alkali metals, even in compound form, requires that these compounds be prepared in high vacuum. Since these are used as photoemissive materials, inside photomultiplier tubes, this is not a problem in use, but detailed studies of properties and even composition are difficult k Usually, such materials are made by allowing alkali vapor to come in contact with a thin film of the group V or VI element. Ffar crystallographic studies, however, powders have been used. Temperatures from 130 to 240°C have been used the alkali vapor has been released from a compound such as the... 

The convenient photon detectors discussed in the previous section cannot be used to measure infrared radiation because photons of these frequencies lack the energy to cause photoemission of electrons as a consequence, thermal detectors must be used. Unfortunately, the performance characteristics of thermal detectors are much inferior to those of phototubes, photomultiplier tubes, silicon diodes, and photovoltaic cells. 

A miniaturized luminometer consists of four micro-dispensers, four micro-cells, and a photodiode array (Fig. 1(a)). The micro-dispensers consisted of capillary tubes placed in each cell. A high photoemission collecting efficiency was about 7% because the photodiode array was closely positioned under the micro-cells. Bioluminescence from the micro-cells was simultaneously detected with the photodiode array (HAMAMATSU SI 133-01, Japan) placed on a base plate that had in-house-made amplifiers. A multifunctional DAQ (National Instruments PCI-MIO-16XE-50, TX, USA) and National Instruments Lab VIEW 6i were used for... 

A photomultiplier (PM) tube is more sensitive than a phototube for the visible and ultraviolet regions. It consists of a photoemissive cathode, which the photon strikes, and a series of electrodes (dynodes), each at a more positive potential (50 to 90 V) than the one before it. When an electron strikes the photo-emissive surface, a primary electron is emitted (this is the photoelectric effect— Albert Einstein received the 1921 Nobel Prize in Physics for its discovery in 1905, not for the special theory of relativity which he also introduced in 1905—see www.lucidcafe.com/lucidcafe/librarv/96mar/einstein.htmD. The primary electron released from the photoemissive surface is accelerated toward the first dynode. The impact of the electron on the dynode surface causes the release of many secondary electrons, which in turn are accelerated to the next electrode where each secondary electron releases more electrons, and so on, up to about 10 stages of amplication. The electrons are finally collected by the anode. The final output of the photomultiplier tube may, in turn, be electronically amplified. 

The most common detector is the photomultiplier tube (PMT). A PMT is a sealed, evacuated transparent envelope (quartz or glass) containing a photoemissive cathode, an anode, and several additional electrodes called dynodes. The photoemissive cathode is a metal coated with an alkali metal or a mixture of elements (e.g., Na/K/Cs/Sb or Ga/As) that emits electrons when struck by photons. The PMT is a more sophisticated version of a vacuum phototube (Fig. 5.17), which contained only a photoemissive cathode and an anode the photocurrent was hmited to the electrons ejected from the cathode. In the PMT (Fig. 5.18), the additional dynodes multiply the available electrons. The ejected electrons are attracted to a dynode that is maintained at a positive... 

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